Method of fabricating an embossed fluid flow field plate

ABSTRACT

An embossed fluid flow field plate for electrochemical cells comprises two sheets of compressible, electrically conductive material. Each sheet has two oppositely facing major surfaces. At least one of the major surfaces has an embossed surface which has a fluid inlet formed therein. The embossed surface has at least one open-faced channel embossed therein extending from the fluid inlet for conducting pressurized fluid introduced at the fluid inlet. A metal sheet is interposed between each of the compressible sheets. The compressible, electrically conductive sheet preferably comprises graphite foil. A method of fabricating an embossed separator plate for use in conjunction with an electrochemical fuel cell comprises (1) providing two sheets of compressible, electrically conductive sheet material, (2) interposing a metal sheet between each of the compressible sheets, and (3) embossing an open-faced channel in at least one of the surfaces of the sheets facing away from the metal sheet.

FIELD OF THE INVENTION

The present invention relates to electrochemical cells. Moreparticularly, the present invention relates to the embossing ofcompressible, electrically conductive sheets for use as fluid flow fieldor separator plates for electrochemical cells, as well as the method offabricating such embossed fluid flow field or separator plates. Inaddition, the present invention relates to embossed coolant flow fieldplates for electrochemical cells.

BACKGROUND OF THE INVENTION

Electrochemical fuel cells convert fuel and oxidant to electricity andreaction product. In electrochemical fuel cells employing hydrogen asthe fuel and oxygen as the oxidant, the reaction product is water. Suchfuel cells generally employ a membrane electrode assembly ("MEA")consisting of a solid polymer electrolyte or ion exchange membranedisposed between two electrodes formed of porous, electricallyconductive sheet material, typically carbon fiber paper. The MEAcontains a layer of catalyst, typically in the form of finely comminutedplatinum, at each membrane/electrode interface to induce the desiredelectrochemical reaction. The electrodes are electrically coupled toprovide a path for conducting electrons between the electrodes throughan external load.

At the anode, the fuel permeates the porous electrode material andreacts at the catalyst layer to form cations, which migrate through themembrane to the cathode. At the cathode, the oxygen-containing gassupply reacts at the catalyst layer to form anions. The anions formed atthe cathode react with the cations to complete the electrochemicalreaction and form a reaction product.

In electrochemical fuel cells employing hydrogen as the fuel andoxygen-containing air (or substantially pure oxygen) as the oxidant, thecatalyzed reaction at the anode produces hydrogen cations (protons) fromthe fuel supply. The ion exchange membrane facilitates the migration ofhydrogen ions from the anode to the cathode. In addition to conductinghydrogen ions, the membrane isolates the hydrogen-containing fuel streamfrom the oxygen-containing oxidant stream. At the cathode, oxygen reactsat the catalyst layer to form anions. The anions formed at the cathodereact with the hydrogen ions that have crossed the membrane to completethe electrochemical reaction and form liquid water as the reactionproduct.

In conventional fuel cells, the MEA is interposed between twofluid-impermeable, electrically conductive plates, commonly referred toas the anode and the cathode plates, respectively. The plates serve ascurrent collectors, provide structural support for the porous,electrically conductive electrodes, provide means for carrying the fueland oxidant to the anode and cathode, respectively, and provide meansfor removing water formed during operation of the fuel cell. When thechannels are formed in the anode and cathode plates, the plates arenormally referred to as fluid flow field plates. When the anode andcathode plates overlay channels formed in the anode and cathode porousmaterial, the plates are normally referred to as separator plates.

Reactant feed manifolds are generally formed in the anode and cathodeplates, as well as in the MEA, to direct the fuel (typicallysubstantially pure hydrogen or hydrogen-containing reformate from theconversion of hydrocarbons such as methanol or natural gas) to the anodeand the oxidant (typically substantially pure oxygen oroxygen-containing gas) to the cathode via the channels formed in eitherthe fluid flow field plates or the electrodes themselves. Exhaustmanifolds are also generally formed in the anode and cathode plates, aswell as the MEA, to direct unreacted fuel and oxidant, as well as wateraccumulated at the cathode, from the fuel cell.

Multiple fuel cell assemblies comprising two or more anodeplate/MEA/cathode plate combinations, referred to as a fuel cell stack,can be connected together in series (or in parallel) to increase theoverall power output as required. In such stack arrangements, the cellsare most often connected in series, wherein one side of a given fluidflow field or separator plate is the anode plate for one cell, the otherside of the plate is the cathode plate for the adjacent cell, and so on.

Perfluorosulfonic ion exchange membranes, such as those sold by DuPontunder its Nafion trade designation, have been used effectively inelectrochemical fuel cells. Fuel cells employing Nafion-type cationexchange membranes require accumulated water to be removed from thecathode (oxidant) side, both as a result of the water transported acrossthe membrane with cations and product water formed at the cathode fromthe electrochemical reaction of hydrogen cations with oxygen. Anexperimental perfluorosulfonic ion exchange membrane, sold by Dow underthe trade designation XUS 13204.10, appears to have significantly lesswater transported with hydrogen cations across the membrane. Fuel cellsemploying the Dow experimental membrane thus tend to accumulate lesswater on the cathode (oxidant) side, as the accumulated water at thecathode is essentially limited to product water formed from theelectrochemical reaction of hydrogen and oxygen.

A typical prior art fluid flow field plate, exemplified by GeneralElectric Company and Hamilton Standard in a 1984 report for the U.S.Department of Energy (LANL No. 9-X53-D6272-1), included a plurality ofparallel open-faced fluid flow channels formed in a major surface of arigid, electrically conductive plate. The parallel channels extendedbetween an inlet header and an outlet header formed in the plate. Theparallel channels were typically rectangular in cross-section, and about0.03 inches deep and about 0.03 inches wide. The inlet header wasconnected to an opening in the plate through which a pressurizedreactant (fuel or oxidant) stream is supplied. The outlet header wasconnected to an opening in the plate through which the exhaust stream isdischarged from the cell. In operation, the reactant stream ran from theinlet to the inlet header and then to the parallel channels from whichreactant from the stream diffused through the porous electrode materialto the electrocatalytically active region of the MEA. The stream thenflowed to the outlet header and then to the outlet from which it wasexhausted from the fuel cell.

Watkins U.S. Pat. Nos. 4,988,583 and 5,108,849 issued Jan. 29, 1991 andApr. 28, 1992, respectively, describe fluid flow field plates whichinclude a fluid supply opening and a fluid exhaust opening formed in theplate surface. Continuous open-faced fluid flow channels formed in thesurface of the plate traverse the central area of the plate surface in aplurality of passes, that is, in a serpentine manner. Each channel has afluid inlet at one end and a fluid outlet at the other end. The fluidinlet and outlet of each channel are directly connected to the fluidsupply opening and fluid exhaust opening, respectively. The continuouschannel design promotes the forced movement of water through eachchannel before the water can coalesce, thereby promoting uniformreactant flow across the surface of the cathode.

U.S. patent application Ser. No. 07/975,791, filed Nov. 13, 1992, nowabandoned, incorporated by reference herein in its entirety, describesand claims a fluid flow field plate for electrochemical fuel cells inwhich the inlet and outlet flow channels are discontinuous. Theemployment of discontinuous flow channels, as described in U.S. patentapplication Ser. No. 07/975,791, now abandoned, has several advantages:

1. Improved performance, particularly at higher reactant inletpressures, resulting from (a) more effective water removal due to betteraccess of the reactant stream to the electrocatalytically active regionat the membrane/electrode interface, (b) more uniform current densitydue to more even distribution of the reactant stream across theelectrocatalytically active area of the fuel cell and the avoidance ofwater pooling in the flow channels, and (c) lower flow fieldplate/electrode contact resistance due to the use of a decreased amountof the flow field plate surface to accommodate the flow channels.

2. Improved fuel cell lifetime resulting from (a) the ability to reducethe compressive load on the electrodes due to decreased contactresistance between the flow field plates and the electrodes, and (b)more uniform reactant gas relative humidity due to more evendistribution of the reactant stream across the electrocatalyticallyactive area of the fuel cell.

3. Reduced manufacturing costs resulting from (a) the ability to reducethe amount of graphite plate milling required for continuous channelsand to relax the tolerances required for the channel dimensions, (b) awider range of materials and fabrication techniques permitted with thediscontinuous flow channel design, such as stamping of flow fieldstencils, to be employed, particularly the use of thinner electricallyconductive sheet materials, as the discontinuous channels do not requirethe thickness and rigidity of the electrically conductive plates inwhich continuous, serpentine flow channels are formed, and (c) theability to employ a stenciled graphite foil laminate, thereby reducingthe weight (and cost) associated with rigid graphite flow field plates.

Conventional methods of fabricating fluid flow field plates require theengraving or milling of flow channels into the surface of the rigid,resin-impregnated graphite plates. These methods of fabrication placesignificant restrictions on the minimum achievable cell thickness due tothe machining process, plate permeability, and required mechanicalproperties. For example, the minimum practical thickness for adouble-sided flow field plate is approximately 0.075 inches.

The conventional resin-impregnated graphite plates are expensive, bothin raw material costs and in machining costs. The machining of channelsand the like into the graphite plate surfaces causes significant toolwear and requires significant processing times.

As described and claimed in U.S. patent application Ser. No. 08/024,660(the "'660 application") now U.S. Pat. No. 5,300,370, incorporatedherein in its entirety, fluid flow field plates may also be fabricatedby a lamination process. Specifically, the '660 application disclosesand claims a laminated fluid flow field assembly for an electrochemicalfuel cell which comprises:

a separator layer formed of electrically conductive, substantially fluidimpermeable sheet material, the separator layer having two oppositelyfacing major surfaces;

a stencil layer formed of electrically conductive sheet material, thestencil layer having two oppositely facing major surfaces, the stencillayer having a fluid inlet formed therein and at least one openingformed therein extending between the major surfaces thereof, the atleast one opening in fluid communication with the fluid inlet; and

means for consolidating the separator layer and the stencil layer alongone of their respective major surfaces.

In operation, the separator layer and the stencil layer cooperate toform at least one open-faced channel for conducting pressurized fluidintroduced at the fluid inlet. The separator layer and the stencil layerare consolidated by compression, preferably in combination with anelectrically conductive adhesive. The laminated fluid flow fieldassembly thus comprises two layers which must be properly positioned andaligned prior to consolidation.

It is often difficult and time consuming to properly position and alignthe separator and stencil layers of a laminated fluid flow fieldassembly. The two-layer laminated fluid flow field assembly also addsboth volume and weight to the fuel cell, as compared, for example, to aone-layer fluid flow field assembly. Thus, as with conventionallyfabricated fluid flow field plates, laminated fluid flow fieldassemblies restrict the extent to which cell thickness can be reducedbecause of the minimum thickness each plate or layer must possess topermit milling or engraving (in the case of graphite plates) ordie-cutting (in the case of stencil layers or laminated assemblies).

Accordingly, it is an object of the present invention to provide animproved fluid flow field plate for use in electrochemical cells that isreduced in weight and volume, and that is simpler and less expensive tomanufacture than conventional fluid flow field plates and laminatedfluid flow field assemblies.

Another object of the present invention is to provide an improved fluidflow field plate for use in electrochemical cells that achieves a higherpower density at a lower cost than conventional fluid flow field platesand laminated fluid flow field assemblies.

Still another object of the invention is to provide fluid flow fieldplates having improved sealing capabilities because of the presence ofsealant grooves embossed in the surface of the plate.

A further object of the present invention is to provide improved coolantflow field plates for use in electrochemical cells.

A still further object of the invention is to provide an improved methodof fabricating an embossed fluid flow field plate for use inelectrochemical cells.

SUMMARY OF THE INVENTION

The above and other objects are achieved by an embossed fluid flow fieldplate for use in conjunction with an electrochemical cell. The platecomprises a sheet of compressible, electrically conductive material. Thesheet has two oppositely facing major surfaces, at least one of themajor surfaces comprising an embossed surface. The embossed surface hasa fluid inlet formed therein and further has at least one open-facedinlet channel embossed therein. The at least one embossed inlet channelextends from the fluid inlet. In operation, the at least one embossedinlet channel conducts pressurized fluid introduced at the fluid inlet.

In the preferred embossed fluid flow field plate, the embossed surfacehas an inlet opening formed therein. The inlet opening extends betweenthe major surfaces of the sheet and is in fluid communication with thefluid inlet. The embossed surface preferably has a fluid outlet formedtherein, the embossed surface further having at least one open-facedoutlet channel embossed therein, the at least one embossed outletchannel extending from the fluid outlet, whereby the at least oneembossed outlet channel conducts pressurized fluid to the fluid outlet.

In one preferred embodiment, the at least embossed inlet channel extendsinto the at least one embossed outlet channel to form at least oneembossed continuous channel, whereby pressurized fluid introduced at thefluid inlet is conducted to the fluid outlet along the at least oneinlet channel and the at least one outlet channel. In this embodiment,the at least one embossed continuous channel forms a serpentine patternon the embossed surface.

In another preferred embodiment, the at least one embossed inlet channelis discontinuous with respect to the at least one embossed outletchannel. In this embodiment, the at least one embossed outlet channelpreferably comprises at least two embossed outlet channels, and the atleast one embossed inlet channel and the at least two embossed outletchannels are preferably interdigitated, whereby each adjacent pair ofembossed outlet channels has an embossed inlet channel disposedtherebetween along a substantial portion thereof.

In the preferred embossed fluid flow field plate, the compressible,electrically conductive sheet preferably comprises graphite foil. Inanother preferred embodiment, the compressible, electrically conductivesheet is a laminated assembly comprising at least two layers of graphitefoil and a metal foil layer interposed between each adjacent pair ofgraphite foil layers.

In still another preferred embodiment, the embossed surface further hasat least one open-faced sealant channel embossed therein. The at leastone embossed sealant channel circumscribes the central portion of theembossed surface and accommodates a substantially fluid impermeablesealant material therein, whereby the sealant material fluidly isolatesthe central portion from the atmosphere surrounding the plate.

The above and other objects are also achieved by an electrochemical fuelcell for converting a fluid fuel stream and a fluid oxidant stream to areaction product stream and electrical energy. The fuel cell comprises:

first and second embossed fluid flow field plates, each of the platescomprising:

a separator sheet formed of compressible, electrically conductivematerial, the separator sheet having two oppositely facing majorsurfaces, at least one of the major surfaces comprising an embossedsurface, the embossed surface having a fluid inlet formed therein andfurther having at least one open-faced inlet channel embossed therein,the at least one embossed inlet channel extending from the fluid inlet,whereby the at least one embossed inlet channel conducts pressurizedfluid introduced at the fluid inlet;

a membrane electrode assembly interposed between the first and secondembossed fluid flow field plates, the membrane electrode assemblycomprising:

an anode having a catalyst associated therewith to render a region ofthe anode electrocatalytically active wherein cations are produced fromthe fluid fuel stream;

a cathode having a catalyst associated therewith to render a region ofthe cathode electrocatalytically active wherein an electrochemicalreaction between the cations and the fluid oxidant stream is promoted;

a solid polymer ion exchange membrane disposed between the anodeassembly and the cathode assembly, the membrane facilitating themigration of cations from the anode assembly to the cathode assembly andisolating the fluid fuel stream from the fluid oxidant stream;

an electrical path for conducting electrons formed at the anode assemblyto the cathode assembly.

The preferred fuel cell further comprises a coolant flow field plateadjacent one of the separator sheets on the side facing away from themembrane. The coolant flow field plate comprises:

a sheet of compressible, electrically conductive, substantially fluidimpermeable material, the sheet having two oppositely facing majorsurfaces, at least one of the major surfaces comprising an embossedsurface, the embossed surface having at least one open-faced sealantchannel embossed therein, the at least one embossed sealant channelcircumscribing the central portion of the embossed surface andaccommodating a substantially fluid impermeable sealant materialtherein, whereby the sealant material fluidly isolates the centralportion from the atmosphere surrounding the plate, the embossed surfacefurther having a coolant inlet, a coolant outlet, and at least oneopen-faced coolant channel formed therein, whereby the at least onecoolant channel conducts pressurized fluid introduced at the coolantinlet toward the coolant outlet.

In the preferred fuel cell, the major surface of the coolant flow fieldplate facing away from the membrane forms a cooling fluid flow fieldplate of an adjacent fuel cell. In the preferred fuel cell, thecompressible, electrically conductive sheet comprises graphite foil.

The above and other objects are further achieved by an embossed coolantflow field plate for use in conjunction with an electrochemical cell.The plate comprises:

a sheet of compressible, electrically conductive, substantially fluidimpermeable material, the sheet having two oppositely facing majorsurfaces, at least one of the major surfaces comprising an embossedsurface, the embossed surface having at least one open-faced sealantchannel embossed therein, the at least one embossed sealant channelcircumscribing the central portion of the embossed surface andaccommodating a substantially fluid impermeable sealant materialtherein, whereby the sealant material fluidly isolates the centralportion from the atmosphere surrounding the plate, the embossed surfacefurther having a coolant inlet, a coolant outlet, and at least oneopen-faced coolant channel formed therein, whereby the at least onecoolant channel conducts pressurized fluid introduced at the coolantinlet toward the coolant outlet.

In the preferred embossed coolant flow field plate, the at least onecoolant channel is embossed in the embossed surface. The at least onecoolant channel can also be milled or die-cut in the embossed surface.In the preferred embossed coolant flow field plate, the compressible,electrically conductive sheet comprises graphite foil.

The above and other objects are still further achieved by an embossedseparator plate for use in conjunction with an electrochemical cell. Theplate comprises:

a sheet of compressible, electrically conductive material, the sheethaving two oppositely facing major surfaces, at least one of the majorsurfaces comprising an embossed surface, the embossed surface having atleast one open-faced sealant channel embossed therein, the at least oneembossed sealant channel circumscribing the central portion of theembossed surface and accommodating a substantially fluid impermeablesealant material therein, whereby the sealant material fluidly isolatesthe central portion from the atmosphere surrounding the plate.

In the preferred separator plate, the embossed surface has a fluid inletformed therein and at least one open-faced inlet channel formed therein.The at least one inlet channel extends from the fluid inlet, whereby theat least one inlet channel conducts pressurized fluid introduced at thefluid inlet. The embossed surface preferably has a fluid outlet formedtherein, and further has at least one open-faced outlet channel formedtherein. The at least one outlet channel extends from the fluid outlet,whereby the at least one outlet channel conducts pressurized fluid tothe fluid outlet. The inlet channel and the outlet channel are mostpreferably embossed in the embossed surface. The inlet channel and theoutlet channel can also be milled or die-cut in the embossed surface.

As in the preferred embossed fluid flow field plate, the inlet andoutlet channels can be continuous, preferably arranged in a serpentinepattern on the embossed surface, or discontinuous, preferably arrangedin an interdigitated pattern on the embossed surface.

In the preferred separator plate, the compressible, electricallyconductive sheet comprises graphite foil. In another preferredembodiment, the compressible, electrically conductive sheet is alaminated assembly comprising at least two layers of graphite foil and ametal foil layer interposed between each adjacent pair of graphite foillayers.

The above and other objects are yet further achieved by an embossedseparator plate for use in conjunction with an electrochemical cell. Theplate comprises:

a sheet of compressible, electrically conductive material, the sheethaving two oppositely facing major surfaces and at least one manifoldopening formed therein between the major surfaces, at least one of themajor surfaces comprising an embossed surface, the embossed surfacehaving at least one open-faced sealant channel embossed therein, the atleast one embossed sealant channel circumscribing the at least onemanifold opening and accommodating a substantially fluid impermeablesealant material therein, whereby the sealant material fluidly isolatesthe at least one manifold opening from the atmosphere surrounding theplate.

The above and other objects are also achieved by a method of fabricatingan embossed separator plate for use in conjunction with anelectrochemical cell. The method comprises:

providing a sheet of compressible, electrically conductive sheetmaterial, the sheet having two oppositely facing major surfaces;

embossing at least one open-faced channel in at least one of the majorsurfaces.

The preferred method further comprises:

forming a fluid inlet on at least one of the major surfaces such thatthe at least one channel extends from the fluid inlet,

whereby the at least one channel conducts pressurized fluid introducedat the fluid inlet.

In the preferred method, the at least one embossed channel circumscribesthe central portion of the at least one major surface and accommodates asubstantially fluid impermeable sealant material in the at least oneembossed channel, whereby the sealant material fluidly isolates thecentral portion from the atmosphere surrounding the plate. The mostpreferred method further comprises:

forming a fluid inlet on at least one of the major surfaces;

forming at least one open-faced channel in the at least one of the majorsurfaces, the at least one channel extending from the fluid inlet,whereby the at least one channel conducts pressurized fluid introducedat the fluid inlet.

The at least one channel is preferably embossed in the at least onemajor surface. The at least one channel can also be milled or die-cut inthe at least one major surface. In the preferred method, thecompressible, electrically conductive sheet comprises graphite foil.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevation view of a fuel cell stack showing theelectrochemically active and humidification sections.

FIG. 2 is an exploded side view of a fuel cell including a membraneelectrode assembly interposed between two conventional fluid flow fieldplates having reactant flow channels formed in the major surfaces facingthe electrodes.

FIG. 3 is an exploded side sectional view of a fuel cell including amembrane electrode assembly interposed between two laminated fluid flowfield assemblies, one layer of which is a stencil layer having openingsformed therein and the other layer of which is a separator layer.

FIG. 4a is a top plan view of one major surface of an embossed fluidflow field plate in accordance with the present invention.

FIG. 4b is a top plan view of the other major surface of the embossedfluid flow field plate illustrated in FIG. 4a.

FIG. 5a is a top plan view of one major surface of an embossed separatorplate in accordance with the present invention.

FIG. 5b is a top plan view of the other major surface of the embossedseparator plate illustrated in FIG. 5a.

FIG. 6 is a side view of a fuel cell incorporating an embossed fluidflow field plate in accordance with the present invention.

FIG. 7a is a top plan view of one major surface of a coolant flow fieldplate in accordance with the present invention.

FIG. 7b is a top plan view of the other major surface of the coolantflow field plate illustrated in FIG. 7a.

FIG. 8 is a plot of cell voltage as a function of current density fortwo fuel cells (designated "A" and "B"). Plot A shows the performance ofa fuel cell employing conventional fluid flow field plates formed ofsolid graphite, such as that illustrated in FIG. 2. Plot B shows theperformance of a fuel cell employing embossed fluid flow field plates,such as those illustrated in FIGS. 4a and 4b.

FIG. 9 is a plot of cell voltage as a function of current density fortwo fuel cells (designated "C" and "D") at 30/30 psig (air/H₂). Plot Cshows the performance of a fuel cell employing an embossed coolant flowfield plate. Plot D shows the performance of a fuel cell employing aconventional coolant flow field plate formed of solid graphite.

FIG. 10 is a plot of cell voltage as a function of current density for afuel cell (designated "E") at 30/30 psig (air/H₂). The plot of FIG. 10shows the performance of a fuel cell employing both embossed fluid flowfield plates and an embossed coolant flow field plate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning first to FIG. 1, a fuel cell stack assembly 10 includes anelectrochemically active section 26 and optionally includes ahumidification section 28. Stack assembly 10 is a modular plate andframe design, and includes a compression end plate 16 and a fluid endplate 18. An optional pneumatic piston 17, positioned within compressionend plate 16, applies uniform pressure to the assembly to promotesealing. Bus plates 22 and 24 located on opposite ends of active section26 provide the negative and positive contacts, respectively, for theelectrical path conducting current generated by the assembly to anexternal electrical load (not shown). Tie rods 20 extend between endplates 16 and 18 to retain and secure stack assembly 10 in its assembledand consolidated state with fastening nuts 21.

Active section 26 includes, in addition to bus plates 22 and 24, aplurality of fuel cell repeating units 12. Each repeating unit 12consists of a membrane electrode assembly, an anode fluid flow fieldplate, a cathode fluid flow field plate (or alternatively anode andcathode separator layers if the anode and cathode reactant flow channelsare formed in the surfaces of the electrode material) and optionally acoolant jacket, as described in more detail below. In the assemblyillustrated in FIG. 1, the repeating units 12 are electrically coupledin series by virtue of the contact between the electrically conductivelayers which form the flow field plates (or the separator layers) andthe coolant jackets.

Optional humidification section 28 includes a plurality ofhumidification assemblies 14, each assembly 14 consisting of fuel oroxidant reactant flow field plate, a water flow field plate, and a watertransport membrane interposed between the reactant flow field plate andthe water flow field plate. When present, humidification section 28imparts water to the fuel and oxidant streams fed to active section 26,thereby preventing the membranes within the active section from dryingout.

FIG. 2 illustrates a conventional fuel cell 30, which includes amembrane electrode assembly 32 interposed between rigid flow fieldplates 34 and 36. Membrane electrode assembly 32 consists of an ionexchange membrane 42 interposed between two electrodes, namely, anode 44and cathode 46. Anode 44 and cathode 46 are typically formed of porouselectrically conductive sheet material, preferably carbon fiber paper,and have planar major surfaces 44a and 46a, respectively. Electrodes 44and 46 have a thin layer of catalyst material disposed on their majorsurfaces 44a and 46a, respectively, at the interface with membrane 42 torender them electrochemically active.

As shown in FIG. 2, anode flow field plate 34 has at least one openfaced channel 34a engraved, milled or molded in its surface facingmembrane 42. Similarly, cathode flow field plate 36 has at least oneopen faced channel 36a engraved, milled or molded in its major surfacefacing membrane 42. When assembled against the cooperating majorsurfaces of electrodes 44 and 46, channels 34a and 36a form the reactantflow field passages for the fuel and oxidant, respectively.

FIG. 3 illustrates a fuel cell 50 including a membrane electrodeassembly 52 interposed between two laminated fluid flow field assemblies54 and 56. As in the membrane electrode assembly 32 of FIG. 2, membraneelectrode assembly 52 consists of an ion exchange membrane 62 interposedbetween two electrodes, namely, anode 64 and cathode 66. Anode 64 andcathode 66 are also typically formed of porous electrically conductivesheet material, preferably carbon fiber paper, and have planar majorsurfaces 64a and 66a, respectively. Electrodes 64 and 66 have a thinlayer of catalyst material disposed on their major surfaces 64a and 66a,respectively, at the interface with membrane 62 to render themelectrochemically active.

As shown in FIG. 3, anode fluid flow field plate 54 is a laminatedassembly of stencil layer 74 and separator layer 76. Similarly, cathodefluid flow field plate 56 is a laminated assembly of stencil layer 84and separator layer 86.

FIG. 4a shows one major surface of an embossed reactant fluid flow fieldplate 100 in accordance with the present invention. Plate 100 ispreferably formed from compressible, electrically conductive graphitefoil sheet material commercially available under the trade designation"GRAFOIL" from UCAR Carbon Company, Inc. of Cleveland, Ohio. GRAFOILsheets are available in standard thicknesses (1/32 inch, 1/16 inch and1/8 inch, for example), but other thicknesses of graphite foil sheetscan be employed as well. The most preferred form of GRAFOIL for use infabricating the embossed fluid flow field plates of the presentinvention is UCAR's grade "GH-A" gasket laminate, and the most preferredthickness is approximately 0.064 inches.

Other suitable electrically conductive materials, sufficiently soft soas to permit embossing, could be used to fabricate the embossed fluidflow field plates described and claimed herein. Such other materialsinclude porous, electrically conductive sheet materials, such as carbonfiber paper, as well as corrosion resistant metals such as niobium,somewhat corrosive resistant materials such as magnesium or copper,particularly when plated with noble metals such as gold or platinum torender them unreactive, and composite materials composed of a corrosiveresistant metal powder, a base metal powder plated with the corrosiveresistant metal, and/or other chemically inert electrically conductivepowders, such as graphite and boron carbide, bonded together with asuitable binder to produce a compressible, electrically conductive sheetmaterial.

As shown in FIG. 4a, a sheet 101, formed of compressible, electricallyconductive material, preferably graphite foil, has a major surface 102.Fluid inlet opening 104 and fluid outlet opening 106 are die-cut andextend between the major surface 102 and its oppositely facing majorsurface 103 shown in FIG. 4b. Other manifold openings 108 are alsodie-cut and extend between the oppositely facing major surfaces of sheet101.

Major surface 102 of plate 100 has embossed therein a reactant fluidflow field 110 comprising a pair of continuous fluid flow field channels114, as well as a sealant channel 112. Each flow field channel 114 has afluid inlet 116 at one end and a fluid outlet 118 at the other end.Fluid inlet 116 is directly connected to fluid inlet opening 104.Similarly, fluid outlet 118 is directly connected to a fluid outletopening 106. Fluid inlet opening 104 is connected to a source of fuel(not shown) in the case of the anode flow field plate or a source ofoxidant (not shown) in the case of the cathode 10 flow field plate.Channel 114 traverses the major central area of major surface 102, in aplurality of passes (two passes in the embodiment illustrated in FIG.4a), which in turn generally corresponds to the electrocatalyticallyactive region of the anode or cathode, which is adjacent when the cellis assembled. Openings 108 serve as manifolds for the various fluidreactant and coolant streams within the cell.

Fluid flow field channels 114 are preferably embossed in sheet 101 usinga die, such as a graphite plate, which has the reverse image of thedesired flow field and sealing groove. Channels 114 are generallyU-shaped or V-shaped in cross-section, the V-shaped channelsfacilitating die release in certain embossing applications. The graphitefoil sheet is embossed at an embossing pressure sufficient to impart,into the compressible sheet material, smooth-surfaced channels, ofsubstantially uniform depth, and having a clean, reverse image of theembossing die. Different flow field patterns and plate sizes willrequire different embossing pressures. The bulk of the sheet material(that is, the portions of the sheet material located apart from thechannels) can also be compressed during the embossing operation. In thisregard, the embossing pressure can be selected to provide theappropriate channel depth and cross-sectional profile, and also toimpart the appropriate electrical conductivity and porosity to the bulkmaterial.

FIG. 5a shows one major surface of an embossed separator plate 120 inaccordance with the present invention. Plate 120 is preferably comprisescompressible, electrically conductive sheet material, preferablygraphite foil. Sheet 121 has a major surface 122. Fluid inlet opening124 and fluid outlet opening 126 are die-cut and extend between themajor surface 122 and its oppositely facing major surface 123 shown inFIG. 5b. Other manifold openings 128 are also die-cut and extend betweenthe oppositely facing major surfaces of plate 121.

Major surface 122 of sheet 121 has embossed therein an open-facedsealant channel 125. Fluid inlet opening 124 is connected to a source offuel (not shown) in the case of the anode flow field plate or a sourceof oxidant (not shown) in the case of the cathode flow field plate.Pressurized reactant fluid introduced at the inlet opening 124 isdirected to the central electrocatalytically active region of the anodeor cathode, which is adjacent when the cell is assembled, via channelsformed in the adjacent anode or cathode or via the interstitial spaceswithin the porous sheet material from which the adjacent anode orcathode is preferably formed. Openings 128 serve as manifolds for thevarious fluid reactant and coolant streams within the cell.

Sealant channel 125 is preferably embossed in sheet 121 using a die, ina manner similar to which the sealant channel 112 and the reactant fluidflow field channel 114 are embossed in sheet 101 illustrated in FIG. 4a.As shown in FIG. 5a, sealant channel 125 circumscribes the centralportion of surface 122, as well as openings 128. Sealant channel 125accommodates a substantially fluid impermeable sealant material (notshown) disposed in channel 125. In operation, the sealant materialfluidly isolates the central portion of surface 122 from the atmospheresurrounding plate 120. As shown in FIG. 5b, sealant channel 127 insurface 123 is structurally and functionally identical to channel 125 insurface 122.

The separator plate 120 illustrated in FIGS. 5a and 5b has no reactantfluid flow field channels embossed, milled, die-cut or otherwise formedtherein. It will be understood, however, that such channels could beformed in either or both of the major surfaces 122, 123 of sheet 121 toprovide a fluid flow field plate similar to that illustrated in FIGS. 4aand 4b.

FIG. 6 shows an exploded side view of a fuel cell repeating unit, suchas repeating unit 12 in FIG. 1, employing embossed reactant and coolantflow field plates. As shown in FIG. 6, a unit cell repeating unit 130 ismade up of an embossed cathode (oxidant) fluid flow field or separatorplate 132, a membrane electrode assembly (MEA) 134, an anode (fuel)fluid flow field or separator plate 136, and a coolant flow field plateor water jacket 138.

The embossed reactant (fuel and oxidant) and coolant flow field plates132, 136, 138 of the fuel cell repeating unit 130 of FIG. 6 arepreferably formed from graphite foil sheets. The unit cell has athickness of approximately 0.200 inches and a unit cell weight ofapproximately 218 grams. Reduced cell weight and volume are achievedwith these graphite foil sheets, which are considerably thinner andlighter than conventional milled graphite fluid flow field plates. Theembossed fluid flow field plates thus drastically reduce the weight andvolume of the fuel cell stack.

Although embossed coolant flow field plates may be employed as describedabove, die-cut coolant flow channels and embossed sealant channels canalso be formed in the coolant flow field plates. FIGS. 7a and 7b areviews, respectively, of a double-sided coolant flow field plate 150having die-cut coolant flow channels 157, 167 and embossed sealantchannels 159, 169 formed therein. Die-cut coolant flow channels 157, 167extend between the major surfaces of plate 150. Plate 150 preferablycomprises compressible, electrically conductive sheet material, mostpreferably graphite foil. Channel 157 has associated therewith a coolantinlet 152 and a coolant outlet 154. A coolant inlet 152 is in fluidcommunication with the coolant inlet manifold opening 156. The coolantoutlet 154 is in fluid communication with the coolant outlet manifoldopening 157. In operation, the coolant flow field plate is assembled andsealed against a fluid flow field plate or separator plate of theadjacent MEAs. Each adjacent fluid flow field plate or separator platecooperates with channels 157, 167 to form a four-sided passage forcontaining and directing coolant fluid within channels 157, 167. Usingthe double-sided design, a single coolant flow field plate can conductcoolant fluid on each of its major surfaces, and simultaneously cool theanode and cathode of neighboring MEAs.

The fluid flow field in the embossed plates described herein maycomprise a single or a plurality of continuous or discontinuous reactantflow channels die-cut or embossed therein. Conventional fluid flow fieldplates having continuous, serpentine channels are described andillustrated in Watkins U.S. Pat. Nos. 4,988,583 and 5,108,849. Fluidflow field plates having discontinuous, interdigitated channels aredescribed and illustrated in U.S. patent application Ser. No. 07/975,791now abandoned. While the present embossed fluid flow field plates areparticularly suited to the discontinuous, interdigitated flow channelsdescribed in U.S. patent application Ser. No. 07/975,791, now abandoned,the embossed configuration can also be employed in conjunction withcontinuous, serpentine flow channel configurations, such as thosedescribed in Watkins U.S. Pat. Nos. 4,988,583 and 5,108,849.

The embossed fluid flow field plates described and illustrated hereincontain fluid manifolds. It will be understood, however, that othermeans for introducing fluids to and discharging fluids from the surfaceof the assemblies are possible. For example, external manifolds may bepreferred in some instances to introduce fluids through an inlet (orinlets) located along an edge (or edges) of the embossed fluid flowfield plates and to discharge fluids from the surface of the platesthrough an outlet (or outlets) located along another edge (or edges) ofthe plates. The embossed fluid flow field described and claimed hereinextend to fuel cells employing such other external manifold designs.

The performance of electrochemical fuel cells incorporating embossedfluid flow field compares favorably to the performance of conventionalfuel cells employing milled graphite fluid flow field plates. FIG. 8 isa plot of cell voltage as a function of current density for two fuelcells (designated "A" and "B") at 30/30 psig (air/H₂). In each of plotsA and B, a milled graphite anode flow field plate was employed, alongwith the following operating conditions: 2.0/1.5 air/H₂ stoichiometry,temperature =75° C., DuPont Nafion 117 cation exchange membrane. In PlotA, a 10-pass, conventional milled graphite cathode flow field plate wasemployed, such as that illustrated in FIG. 2. In Plot B, a 7-pass,embossed graphite foil cathode flow field plate was employed, such asthat illustrated in FIG. 4. FIG. 8 shows that the performance of a fuelcell employing an embossed reactant fluid flow field plate comparesfavorably, and was superior at high current density, to the performanceof a fuel cell employing conventional, milled graphite fluid flow fieldplates.

Similarly, the performance of a fuel cell employing embossed coolantflow field plates compares favorably to the performance of aconventional fuel cell employing milled graphite coolant flow fieldplates. FIG. 9 is a plot of cell voltage as a function of currentdensity for two fuel cells (designated "C" and "D") at 30/30 psig(air/H₂). In each of plots C and D, a milled graphite anode flow fieldplate and a milled graphite cathode flow field plate were employed,along with the following operating conditions: 2.0/1.5 air/H₂stoichiometry, temperature =75° C., DuPont Nafion 117 cation exchangemembrane. Plot C shows the performance of a fuel cell employing embossedcoolant flow field plates. Plot D shows the performance of a fuel cellemploying conventional, milled graphite coolant flow field plates.

In order to minimize cell weight and volume, the electrochemical fuelcell of the present invention preferably employs both embossed reactantfluid flow field plates and embossed coolant flow field plates. FIG. 10is a plot of cell voltage as a function of current density for a fuelcell (designated "E") at 30/30 psig (air/H₂). A 2-pass anode flow fieldplate was employed, along with the following operating conditions:2.0/1.5 air/H₂ stoichiometry, temperature =75° C., Dow experimentalcation exchange membrane. FIG. 10 shows the favorable performanceachieved by a fuel cell employing both embossed reactant fluid flowfield plates and embossed coolant flow field plates, as well as theoperability of the embossed reactant and coolant flow field plates inconjunction with the Dow experimental cation exchange membrane.

While particular elements, embodiments, and applications of the presentinvention have been shown and described, it will be understood, ofcourse, that the invention is not limited thereto since modificationsmay be made by those skilled in the art, particularly in light of theforegoing teachings. It is therefore contemplated by the appended claimsto cover such modifications as incorporate those features which comewithin the spirit and scope of the invention.

What is claimed is:
 1. A method of fabricating an embossed separatorplate for use in conjunction with an electrochemical cell, said methodcomprising:providing two sheets of compressible, electrically conductivesheet material, each of said sheets having two oppositely facing majorsurfaces; interposing a metal sheet between each of said compressiblesheets; embossing at least one open-faced channel in at least one of themajor surfaces of said compressible sheets facing away from said metalsheet.
 2. The method of claim 1 further comprising:forming a fluid inleton at least one of the embossed major surfaces such that said at leastone channel extends from said fluid inlet, whereby said at least onechannel conducts pressurized fluid introduced at said fluid inlet. 3.The method of claim 1 wherein said at least one embossed channelcircumscribes the central portion of said embossed major surfaces andaccommodates a substantially fluid impermeable sealant material in saidat least one embossed channel, whereby said sealant material fluidlyisolates said central portion from the atmosphere surrounding saidplate.
 4. The method of claim 3 further comprising:forming a fluid inleton at least one of the embossed major surfaces; forming at least oneopen-faced fluid channel in said at least one embossed major surface,said at least one fluid channel extending from said fluid inlet, wherebysaid at least one fluid channel conducts pressurized fluid introduced atsaid fluid inlet.
 5. The method of claim 4 wherein said at least onefluid channel is embossed in said at least one embossed major surface.6. The method of claim 4 wherein said at least one fluid channel ismilled in said at least one embossed major surface.
 7. The method ofclaim 4 wherein said at least one fluid channel is die-cut in said atleast one embossed major surface.
 8. The method of claim 1 wherein eachof said two compressible sheets comprises graphite foil.